CLIAMS:We Claim:
1. A system for earth fault protection, the system comprising:
• a synchronous electric machine comprising a stator, a rotor and a field winding;
• an excitation transformer comprising a primary winding and a secondary winding, wherein the excitation transformer power is used to magnetize the synchronous electric machine;
• a controlled rectifier coupled between the excitation transformer and the synchronous electric machine, wherein the controlled rectifier converts alternating current (AC) output of the excitation transformer to direct current (DC) thereby provides DC excitation to the field winding of the synchronous electric machine; and
• an earth fault protection circuitry to detect earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine, wherein the earth fault protection circuitry protects the secondary winding of the excitation transformer and the field winding of the synchronous electric machine after detection of earth fault, wherein the earth fault protection circuitry further comprises:
• a DC injection unit to inject a DC voltage between the controlled rectifier and earth; and
• a decision unit to measure earth resistance thereby detect earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine, wherein earth fault is detected by comparing the measured earth resistance with at least one predetermined threshold value.
2. The system of claim 1, wherein the earth fault protection circuitry further comprises a coupling unit to protect the DC injection unit against overvoltage.
3. The system of claim 2, wherein the coupling unit comprises a plurality of resistors.
4. The system of claim 1, wherein the earth fault protection circuitry further comprises an alarm unit for providing an alarm to indicate earth fault in the secondary winding of the excitation transformer orin the field winding of the synchronous electric machine.
5. The system of claim 4, wherein the alarm is selected from a group consisting of audio, visual, tactile or combination thereof.
6. The system of claim 1, wherein the earth fault protection circuitry further comprises an interrupt unit to cut off the power supply from the excitation transformer to the synchronous electric machine in case the earth fault is detected in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine.
7. The system of claim 6, wherein the interrupt unit is selected from a group consisting of an electronic switch, an electro-mechanical switch, a relay, a programmable logic controller or combination thereof.
8. The system of claim 1 wherein:
• the decision unit activates the alarm unit if the measured earth resistance approaches a first predetermined threshold value; and
• the decision unit is further configured to activate the interrupt unit if the measured earth resistance approaches a second predetermined threshold value.
9. The system of claim 1, wherein the DC injection unit injects the DC voltage in the form of square wave.
10. The system of claim 1, wherein the controlled rectifier is selected from the group consisting of a diode, a thyristor, a transistor and combination thereof.
11. The system of claim 1, wherein the decision unit comprises a microprocessor, a microcontroller or a DSP (Digital signal processor).
12. A method for protecting against an earth fault, the method comprising:
• magnetizing a synchronous electric machine using an excitation transformer;
• converting alternating current (AC) output of the excitation transformer to direct current (DC) using a controlled rectifier thereby providing the DC excitation to a field winding of the synchronous electric machine;
• injecting a DC voltage by an earth fault protection circuitry between the controlled rectifier and earth for measuring earth resistance;
• detecting earth fault in a secondary winding of the excitation transformer or in the field winding of the synchronous electric machine using the earth fault protection circuitry by comparing the measured earth resistance with at least one predetermined threshold value; and
• protecting the secondary winding of the excitation transformer and the field winding of the synchronous electric machine against earth fault using the earth fault protection circuitry after detection of earth fault.
13. The method of claim 12, wherein the injected DC voltage is in the form of square wave.
14. The method of claim 12, wherein the step of measuring earth resistance further comprises:
• measuring a circulating current produced by the injected DC voltage; and
• computinga circulating current difference and an injected DC voltage difference by using their measured values at different time intervals for measuring earth resistance.
15. The method of claim 12, wherein the step of protecting against earth fault further comprises:
• activating an alarm unit by the decision unit for providing an alarm to indicate earth fault if the measured earth resistance approaches a first predetermined threshold value; and
• activating an interrupt unit by the decision unit to cut off the power supply between the excitation transformer and the synchronous electric machine if the measured earth resistance approaches a second predetermined threshold value.
16. The method of claim 15, wherein the second predetermined threshold value is less than the first predetermined threshold value.
17. The method of claim 15, wherein activating, by the alarm unit, the alarm to indicate earth fault in the secondary winding of the excitation transformer or in the field winding of synchronous machine
18. The method of claim 17, wherein the alarm is selected from a group consisting of audio, visual, tactile or combination thereof.
19. The method of claim 12, wherein the earth fault protection circuitry is protected against overvoltage by using a coupling unit, wherein the coupling unit comprises a plurality of resistors.
20. A system for earth fault protection, the system comprising:
• a synchronous electric machine comprising a stator, a rotor and a field winding;
• an excitation transformer comprising a primary winding and a secondary winding, wherein the excitation transformer power is used to magnetize the synchronous electric machine;
• a controlled rectifier coupled between the excitation transformer and the synchronous electric machine, wherein the controlled rectifier converts alternating current (AC) output of the excitation transformer to (DC) direct current thereby provides DC excitation to the field winding of the synchronous electric machine; and
• an earth fault protection circuitry to detect earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine, wherein the earth fault protection circuitry protects the secondary winding of the excitation transformer and the field winding of the synchronous electric machine after detection of earth fault, wherein the earth fault protection circuitry further comprises:
• a DC injection unit to inject a DC voltage in the form of square wave between the controlled rectifier and earth;
• a coupling unit to protect the DC injection unit against overvoltage;
• a decision unit to measure earth resistance thereby detect earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine, wherein earth fault is detected by comparing the measured earth resistance with at least one predetermined threshold value;
• an alarm unit for providing an alarm to indicate earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine; and
• an interrupt unit to cut off the power supply from the excitation transformer to the synchronous electric machine in case earth fault is detected in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine.
21. The system of claim 20 wherein:
• the decision unit activates the alarm unit if the measured earth resistance approaches a first predetermined threshold value.
• the decision unit is further configured to activate the interrupt unit if the measured earth resistance approaches a second predetermined threshold value.
22. The system of claim 20, wherein the alarm unit activates the alarm to indicate earth fault in the secondary winding of the excitation transformer or in the field winding of synchronous machine.
23. The system of claim 22, wherein the alarm is selected from a group consisting of audio, visual, tactile or combination thereof.
24. The system of claim 20, wherein the coupling unit comprises a plurality of resistors.
25. The system of claim 20, wherein the controlled rectifier is selected from the group consisting of a diode, a thyristor, a transistor and combination thereof.
26. The system of claim 20, wherein the decision unit is a microprocessor, a microcontroller or a DSP (Digital signal processor).
27. The system of claim 20, wherein the interrupt unit is selected from a group consisting of an electronic switch, an electro-mechanical switch, a relay, a programmable logic controller or combination thereof. ,TagSPECI:FIELD OF INVENTION
The present invention relates to a system and method for detecting earth fault in field winding of a synchronous machine as well as in secondary winding of an excitation transformer. The system and method of the present invention protects the excitation transformer and the field winding of synchronous machine against earth fault.

BACKGROUND
Existing systems employ a separate circuitry for earth fault detection in each of the excitation transformers and the field winding of the synchronous machine which leads to increase in power, space and cost. Also the conventional methods employ a complex circuitry for earth fault detection that further increases cost and space for implementation of the circuitry. Further, the existing methods and systems do not provide integrated protection system for field winding of synchronous machine and secondary winding of the excitation transformer against earth fault. Hence, there is a need for a system and method that solves the above mentioned problems of the prior art systems.

SUMMARY OF THE INVENTION
It is an object of the present invention to detect earth fault infield winding of a synchronous machine as well as in secondary winding of an excitation transformer circuit using a single circuitry.
It is another object of the invention to provide a single circuitry which detects earth fault in a field winding of synchronous machine as well as in an excitation transformer by injecting a DC voltage. Further, the circuitry protects the synchronous machine field winding and the excitation transformer when earth fault is detected.
It is further object of the present invention to provide an earth fault protection circuitry. The earth fault protection circuitry detects occurrence of earth fault in field winding of the synchronous machine as well as circuit of excitation transformer secondary winding.
It is yet another object of the present invention to provide a system for earth fault detection and protection, the system comprising:
• a synchronous electric machine comprising a stator, a rotor and a field winding;
• an excitation transformer comprising a primary winding and a secondary winding, wherein the excitation transformer power is used to magnetize the synchronous electric machine;
• a controlled rectifier coupled between the excitation transformer and the synchronous electric machine, wherein the controlled rectifier converts alternating current (AC) output of the excitation transformer to direct current (DC) thereby provides DC excitation to the field winding of the synchronous electric machine; and
• an earth fault protection circuitry to detect earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine, wherein the earth fault protection circuitry protects the secondary winding of the excitation transformer and the field winding of the synchronous electric machine after detection of earth fault, wherein the earth fault protection circuitry further comprises:
• a DC injection unit to inject a DC voltage between the controlled rectifier and earth;
• a coupling unit to protect the DC injection unit against overvoltage; and
• a decision unit to measure earth resistance thereby detect earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine, wherein earth fault is detected by comparing the measured earth resistance with at least one predetermined threshold value.
In accordance with one embodiment, the earth fault protection circuitry further comprises:
• an alarm unit for providing an alarm to indicate earth fault in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine; and
• an interrupt unit to cut off the power supply from the excitation transformer to the synchronous electric machine in case earth fault is detected in the secondary winding of the excitation transformer or in the field winding of the synchronous electric machine.
It is yet another object of the present invention to provide a method for protecting against an earth fault, the method comprises:
• magnetizing a synchronous electric machine using an excitation transformer;
• converting alternating current (AC) output of the excitation transformer to direct current (DC) using a controlled rectifier thereby providing the DC excitation to a field winding of the synchronous electric machine;
• injecting a DC voltage by an earth fault protection circuitry between the controlled rectifier and earth for measuring earth resistance;
• detecting earth fault in a secondary winding of the excitation transformer or in the field winding of the synchronous electric machine using the earth fault protection circuitry by comparing the measured earth resistance with at least one predetermined threshold value; and
• protecting the secondary winding of the excitation transformer and the field winding of the synchronous electric machine against earth fault using the earth fault protection circuitry machine after detection of earth fault.
In accordance with one embodiment, the earth fault protection circuitry is protected against overvoltage by using a coupling unit that may comprise a plurality of resistors.
According to one embodiment of the present invention, the step of measuring earth resistance further comprises:
• measuring a circulating current produced by the injected DC voltage; and
• computing a circulating current difference and an injected DC voltage differenceby using their measured values at different time intervals for measuring earth resistance.
According to one embodiment of the present invention, the step of protecting against earth fault further comprises:
• activating an alarm unit by the decision unit for providing an alarm to indicate earth fault if the measured earth resistance approaches a first predetermined threshold value; and
• activating an interrupt unit by the decision unit to cut off the power supply between the excitation transformer and the synchronous electric machine if the measured earth resistance approaches a second predetermined threshold value, wherein the second predetermined threshold value is less than the first predetermined threshold value.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:
Figure 1 is a schematic diagram illustrating a system for providing earth fault protectionin accordance with the present invention;
Figure 2 illustrates an equivalent circuit for earth fault protection ofthe synchronous machine and excitation transformer;
Figure 3 illustrates the simplified equivalent circuit for the circuit shown inFigure 2;
Figure 4 illustrates waveforms showing the effect of the earth fault on the measured voltage UM.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are best understood by reference to the detailed figures and description set forth herein. These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.
Reference throughout this specification to "one embodiment",“another embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment", "in an embodiment" or"in another embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
Embodiments presented herein describe techniquesfor detecting earth fault inan excitation transformer as well as ina synchronous machine. The techniques include an earth fault protection circuitry for detection and protection of the excitation transformer as well asthesynchronous machine against earth fault.
Figure 1 illustrates a system 100 for providing earth fault protection,in accordance with an exemplary embodiment of the present invention.It should be noted that those skilled in art will recognize that the system 100 has been simplified to better illustrate features of the present invention. The system 100comprises: a synchronous machine 101, an excitation circuit 102 and an earth fault protection circuitry 103.
The synchronous machine 101comprises a rotor 111, a field winding 112, a stator 113, and an armature winding (not shown in FIGURE). Synchronous machines can be, such as but not limited to, synchronous generators, synchronous motors, and power factor compensators.
The excitation circuit 102 comprises an excitation transformer 121, a controlled switch 122 and a controlled rectifier 123. The excitation transformer 121includes a primary winding 124 and a secondary winding 125.The excitation transformer 121is fed with a high voltageAC power supply 126. The excitation transformer 121 steps down this high voltageAC power into a low voltage AC power which is obtained at the secondary winding 125 of the excitation transformer 121.
A controlled rectifier 123 is connected at the output of the secondary winding 125 of the excitation transformer 121. The controlled rectifier 123 converts the AC output of the excitation transformer 121 to a DC voltage127. The DC excitation to the field winding 112 of the synchronous machine 101 is provided by feeding the DC voltage 127. The controlled rectifier 123 can be for example, without limitation, a diode, a thyristor, a transistor or combination thereof. If thyristors are used for the purpose of rectification, then some suitable firing circuitry is used for firing of gate terminals of thyristors.
In an embodiment of the invention,a controlled switch 122 as shown in Figure1, is connected between the secondary winding 125 and the controlled rectifier 123 which disconnects the excitation transformer 121 from remaining parts of the system 100 in case earth fault is detected by the earth fault protection circuitry 103. The earth fault protection circuitry 103 controls the controlled switch 122 when earth fault is detected in the secondary winding 125 of the excitation transformer 121 or in the field winding 112 of the synchronous machine 101, thereby providing protection to the system 100 and other circuitry associated with it.
The earth fault protection circuitry 103with reference to Figure 1, detects the earth fault in the secondary winding 125 of the excitation transformer 121or in the field winding 112 of the synchronous machine 101. The earth fault protection circuitry comprises a DC injection unit 131,a coupling unit 132, a decision unit 133, an interrupt unit 134 and an alarm unit 135.
The earth fault protection circuitry 103 is connected between the controlled rectifier 123 and the field winding 112 through a coupling unit 132. The DC injection unit 131 continuously injects a DC voltage 136 between the controlled rectifier 123 and the field winding 112 of the synchronous machine 101.
In one embodiment of the invention, the coupling unit 132 comprises two high value resistances RV1 and RV2 connected in parallel to each other as shown in Figure 1.The coupling unit 132 protects the DC injection unit 131 against overvoltage.
In further embodiment of the invention, the DC injection unit 131 injects the DC voltage 136 in form of square wave. The DC voltage 136 may range from ± 15 or ± 24 Volts which depends on the type of the earth fault protection circuitry 103. In order to improve accuracy,frequency of the injected DC voltage 136 can be set from 0.1 Hz to 1.0 Hz. The DC injection unit 131 can be, such as but not limited to, a square wave generator, a multi-vibrator or a device that can provide output in the form of square wave.
In an embodiment, the decision unit 133 may comprise a memory unit, a processing unit, a power supply and a control logic (not shown).The processing unit can be, such as but not limited to, a microprocessor, a DSP, a microcontroller or combination thereof. The decision unit 133 controls all the functionalities of said earth fault protection circuitry 103. The memory unit (not shown) can be external or internal to the decision unit 133. The memory unit can be any storage device for example, without limitation, RAM, ROM, EPROM, EEPROM, DRAM, SRAM, flash memory, hard disk,magnetic storage or like.
The decision unit 133 measures earth resistance (REequ) continuously and compares measured earth resistance with atleast one predetermined threshold value stored in the memory unit (not shown) .
The decision unit also controlsthe interrupt unit 134 and the alarm unit 135.If the earth resistance (REequ) of the equivalent circuit (described in Figure 2) evaluated by the decision unit 133approaches a first predetermined threshold value,then the decision unit 133 activates the alarm unit 135 to indicate that earth fault has been occurred in the secondary winding 125 of the excitation transformer 121or in the field winding 112 of the synchronous machine 101.Further if the measured earth resistance (REequ) approaches a second predetermined threshold value, then the decision unit 133 activates the interrupt unit 134 to provide protection against earth fault. The interrupt unit 134 disconnects the excitation transformer 121 from the remaining parts of the system 100 by cutting-off the output supply from the secondary winding 125 of the excitation transformer 121 as shown in Figure 1. By disconnecting the excitation transformer 121 from the remaining parts of the system 100, the earth fault protection circuitry 103 ensures the protection of the system 100 and other external circuitry associated with it against earth fault and hence prevents from burn out or other type of damages. The interrupt unit 134 can be, such as but not limited to, a logic circuit, a processor, a programmable logic controller or combination thereof.In one embodiment of the invention, single processing unit is used for decision unit 133 and interrupt unit 134.
The alarm unit 135provides an alarm to indicate the presence of earth fault in the secondary winding 125 of the excitation transformer 121 or in the field winding 112 of the synchronous machine 101.The decision unit continuously measures earth resistance and compares the measured earth resistance with a first predetermined threshold value and based on the comparison, it further activates the alarm unit 135 so as to provide the alarm thereby indicating earth fault in the secondary winding 125 of the excitation transformer 121 or in the field winding 112 of the synchronous machine 101.The alarm can be, such as but not limited to, audio, visual, tactile or combination thereof. The alarm may be further transmitted by wired or wireless means to a remote terminal.
The controlled switch 122is connected between the secondary winding 125 of the excitation transformer 121 and the controlled rectifier 123,wherein the controlled switch 122 is in closed position when there is no earth fault. As soon as the decision unit 133 evaluates the occurrence of earth fault, a command is transmitted to the interrupt unit 134 for protecting the system 100 from earth fault.After receiving the command, the interrupt unit 134 disconnects the excitation transformer 121 from the controlled rectifier 123 by changing the position of controlled switch 122 from the closed position to open position, thereby protecting the system 100 against earth fault.In an embodiment of the invention, thecontrolled switch 122 can be such as but not limited to a relay, a switch, an electro-magnetic switch, an electro-mechanical switch, a programmable logic controller or combination thereof.
In another embodiment of the invention, the decision unit 133 controls the injection frequency and injection voltage of the DC injection unit 131 depending on the requirement. Basically the decision unit 133 measures earth resistance after the injection of the DC voltage 136 between the controlled rectifier 123 and the field winding 112 of the synchronous machine 101, then after measuring earth resistance,it compares the measured earth resistance with at least one predetermined threshold value. In case of no earth fault, the earth resistance value is infinite. But in case of earth fault, the earth resistance value decreases. The decision unit 133continuously measures the earth resistance.When measured earth resistance approaches the first predetermined threshold value, the decision unit 133 activates the alarm unit 135 to indicate that earth fault has been occurred in the secondary winding 125 of the excitation transformer 121or in the field winding 112 of the synchronous machine 101. But if themeasured earth resistance approaches the second predetermined threshold value, then the decision unit 133 activates the interrupt unit 134 to provide the protection to all components of system 100 and other external circuitry (not shown in Figure) associated with it against earth fault. The interrupt unit 134 then disconnects the excitation transformer 121 from the remaining parts of the system 100 by cutting-off the output supply from the secondary winding 125 of the excitation transformer 121.
In one embodiment of the invention, the first predetermined threshold value is greater than the second predetermined threshold value.
In one embodiment of the invention, components or parts of the earth fault protection circuitry 103like the coupling unit 132, the DC injection unit 131, the decision unit 133, the interrupt unit 134 and the alarm unit 135 can be integrated into a single chip using suitable chip fabricationtechnology.
Figure 2 illustrates an equivalent circuit 200 for earth fault protectionof the synchronous machine and excitation transformer shown in Figure 1. The equivalent circuit 200 comprises two coupling resistances (201 and 202), a field winding 203, a field earth resistance 204, a field earth capacitance 205, an excitation transformer winding 206, an excitation earth resistance 207, an excitation earth capacitance 208, a DC injection unit 209, a decision unit 210, an alarm unit 211, an interrupt unit 212 and an internal resistance 213. In an embodiment of the invention, the excitation transformer winding 206 refers to the secondary winding 125 and the field winding 203 refers to thefieldwinding 112 of the rotor 111 of the synchronous machine 101 shown in Figure 1.The decision unit 210 is connected to the field winding 203 and the excitation transformer winding 206 through the two coupling resistances RV1 (201) and RV2 (202). The two coupling resistances (201, 202) protect the DC injection unit 209 from overvoltage. The resistances 201, 202 are of high value and are connected in parallel.
Figure 3 illustrates the simplified equivalent circuit for the circuit shown inFigure 2.The equivalentcoupling resistance 301 (RV),as shown in Figure 3, of the two coupling resistances RV1 (201) and RV2 (202)isdescribed using the following equation:
RV= RV1 * RV2/ (RV1 + RV2) (Equation1)
In normal mode of operation when there is no fault in the excitation transformer winding 206 or in the field winding 203, the value of the excitation earth resistance RE(ET) (207) and the value of the field earth resistance RE(F)(204) is infinite. Due to this, these two resistances 204 and 207 will act as an open circuit in case of no earth fault. Therefore, all the current flows through the field earth capacitance CE(F) (205) and excitation earth capacitance CE(ET) (208) and voltage drop occurs only across these two capacitances 205 and 208 when there is no earth fault.
In case when there is earth fault in the excitation transformer winding 206 or in the field winding 203, then there will be some finite value of the excitation earth resistance RE(ET) (207) or the field earth resistance RE(F) (204) depending upon the location of the fault.In one embodiment, when earth fault is only atthe excitation transformer winding 206, the value of the equivalent earth resistance REequis equal to RE(ET). In another embodiment, when earth fault is only at the field winding 203, the value of the equivalent earth resistance is equal to RE(F).
As illustrated in Figure 2, the DC injection unit 209 continuously injects a square wave DC voltage. To equalize the displacement of the field voltage, polarity of the injected DC voltage is periodically changed.The injected square wave DC voltage produces a circulating current (Iaux). The decision unit 210 evaluates value of the circulating current (Iaux). The value of the circulating current at different time intervalsis considered to determine circulating current difference ?Iaux.
Also, the decision unit 210 continuously measures the injected DC voltage. The decision unit 210 considers the value of the injected DC voltage (Uaux)at different time intervals to determine injected DC voltagedifference?Uaux. The value of the injected voltage(Uaux) at different time intervals is considered to determine injected DC voltage difference ?Uaux.
Once the value of ?Uauxand?Iauxis determined, the decision unit 210 calculates the value of the equivalent earth resistance by using the following equation:
REequ = (?Uaux/ ?Iaux)- (RV/2+RM) (Equation 2)
where
REequ = Equivalent Earth Resistance,
RM = Internal Resistance,
RV = Coupling Resistance,
?Uaux = Injected DCVoltage Difference, and
?Iaux = Circulating Current Difference.
The decision unit 210 compares the value of the measured equivalent earth resistance REequwith at least one predetermined threshold value. In an exemplary embodiment, a plurality of predetermined threshold values are stored in the memory unit (not shown)of the decision unit 210. The decision unit 210 performs specific operations depending on the comparison of the equivalent earth resistance REequwith the predetermined threshold values.
The decision unit activates the alarm unit 211 or the interrupt unit 212 depending on comparison of the equivalent earth resistance REequ with the predetermined threshold values. If the measured equivalent earth resistance REequ approaches the first predetermined threshold value, then the decision unit 210 activates alarm unit 211 thereby indicating that earth fault has been occurred in the excitation transformer winding 206 or in the field winding 203. If measured equivalent earth resistance REequ approaches the second predetermined threshold value, then the decision unit 210 activates interrupt unit 212 to cut off the output supply from the excitation transformer winding 206 and thereby protect the system against earth fault. The value of the first predetermined threshold for the alarm unit activation is greater than the second predetermined threshold.
In an exemplary embodiment, the typical value of the first predetermined threshold and the second predetermined threshold are 20K? and 5K? respectively.
Figure 4 shows four waveforms for the applied voltage Uaux(401), measured voltage UM in case of no fault (402), voltage across CEequ403 (VCEequ)and measured voltage UM in case of earth fault (404) respectively.
With Reference to Figure 4, waveform 401 shows the injected DC voltage (Uaux).The injected DC voltage(Uaux) is in the form of square wave.To equalize the displacement of the field voltage, polarity of the injected DC voltage(Uaux) is periodically changed. The frequency range of the injected DC voltage(Uaux)may range from 0.1 to 1 Hz and the amplitude of the injected DC voltage may range from ± 15 to ± 24 Volts.
Waveform 402shows the measured voltage UM in case of no fault. When there is no fault, the value of the equivalent earth resistanceREequ(302 as shown in Figure 3) is infinite and there is no voltage drop across REequ.Therefore, all the voltage drop occurs across the equivalent capacitance CEequ(303). With reference to Figure 2 and Figure 3, the equivalent capacitance CEequ(303) of the field earth capacitance CE(F) (205) and excitation earth capacitance CE(ET) (208) is given by the following equation:
CEequ = CE(F) +CE(ET) (Equation 3)
As shown in Figure 4, waveform 403 describes the charging and discharging of the equivalent earth capacitance CEequin case of no earth fault i.e. when REequ is infinite.For the positivepolarity of the applied voltage Uaux, the equivalent earth capacitance CEequstarts charging through coupling resistance RV (301) and during this time interval, the value of the measured voltage UM starts decreasing as shownin waveform 402. Similarly for the negative polarity of the applied voltageUaux, the capacitor starts discharging as shown in waveform 403 and during this time interval, the value of the measured voltage UM starts increasing as shown in waveform 402.
In case of earth fault in the excitation transformer winding 206 or in the field winding 203, there will be some finite value of the excitation earth resistance RE(ET) (207) and/or the field earth resistance RE(F) (204) with reference to Figure 3. The excitation earth resistance RE(ET) (207) and the field earth resistance RE(F) (204) are in parallel and hence the equivalent earth resistance REequis given as:
REequ = RE(F) * RE (ET)/(RE(F)+RE(ET)) (Equation4)
In one embodiment, when earth fault is only at the excitation transformer winding 206, the value of the equivalent earth resistance is equal to RE(ET). In another embodiment, when earth fault is only at the field winding 203, the value of the equivalent earth resistance is equal to RE(F).
Therefore the voltage drop across CEequin case of earth faultalsodepends upon the value of the earth resistance REequ. Therefore, the voltage across CEequ is given by the following equation:
VCEequ = Umax(1-e?) (Equation 5)
Where,
VCEequis the voltage across equivalent earth capacitance CEequ,
? is time constant, and given by ? = - t / ((RV+REequ)* CEequ),
Umax is maximum injected DC voltage, and
t is time period.
Following equation shows the effect of the equivalent earth resistance REequ and equivalent earth capacitance CEequon the measured voltage UM:
UM = Uaux – VCEequ (Equation6)
With Reference to Figure 4, waveform 404 illustrates effect of the equivalent earth resistance REequ on value of the measured voltage (UM) in case of symmetrical fault. In case of earth fault the REequhas finite value and therefore REequand CEequ forms a parallel RC circuit and due to this the value of the measured voltage UM is effected as shown in waveform 404. The dotted line waveform 404 shows voltage drop across REequ.In an embodiment, in case of an unsymmetrical fault, the measured voltage (UM) waveform either shifts downward or upward.
Although the described method for detection and protection against earth fault for synchronous machines, the embodiments presented herein apply equally to other electrical machines as well. The system described herein may be fully or partially digitaland may be provided in any of a number of configurations. The particular configurations and manners described herein are by way of example and it is understood that the earth fault detection and protection of the present invention may be implemented in a number of different ways for use in a wide variety of configurations and applications.
While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.
Also, techniques, systems, subsystems and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or connected with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.